This variable is a list of lists, where the first element of each of
these lists is the name of the mail group (they do not have to be called
something beginning with ‘mail’, by the way), and the second
element is a regular expression used on the header of each mail to
determine if it belongs in this mail group. The first string may
contain ‘\\1’ forms, like the ones used by replace-match to
insert sub-expressions from the matched text. For instance:

("list.\\1" "From:.* \\(.*\\)-list@majordomo.com")

In that case, nnmail-split-lowercase-expanded controls whether
the inserted text should be made lowercase. See Fancy Mail Splitting.

The second element can also be a function. In that case, it will be
called narrowed to the headers with the first element of the rule as the
argument. It should return a non-nil value if it thinks that the
mail belongs in that group.

The last of these groups should always be a general one, and the regular
expression should always be ‘""’ so that it matches any mails
that haven't been matched by any of the other regexps. (These rules are
processed from the beginning of the alist toward the end. The first rule
to make a match will “win”, unless you have crossposting enabled. In
that case, all matching rules will “win”.) If no rule matched, the mail
will end up in the ‘bogus’ group. When new groups are created by
splitting mail, you may want to run gnus-group-find-new-groups to
see the new groups. This also applies to the ‘bogus’ group.

If you like to tinker with this yourself, you can set this variable to a
function of your choice. This function will be called without any
arguments in a buffer narrowed to the headers of an incoming mail
message. The function should return a list of group names that it
thinks should carry this mail message.

Note that the mail back ends are free to maul the poor, innocent,
incoming headers all they want to. They all add Lines headers;
some add X-Gnus-Group headers; most rename the Unix mbox
From<SPACE> line to something else.

The mail back ends all support cross-posting. If several regexps match,
the mail will be “cross-posted” to all those groups.
nnmail-crosspost says whether to use this mechanism or not. Note
that no articles are crossposted to the general (‘""’) group.

nnmh and nnml makes crossposts by creating hard links to
the crossposted articles. However, not all file systems support hard
links. If that's the case for you, set
nnmail-crosspost-link-function to copy-file. (This
variable is add-name-to-file by default.)

If you wish to see where the previous mail split put the messages, you
can use the M-x nnmail-split-history command. If you wish to see
where re-spooling messages would put the messages, you can use
gnus-summary-respool-trace and related commands (see Mail Group Commands).

Header lines longer than the value of
nnmail-split-header-length-limit are excluded from the split
function.

By default, splitting does not decode headers, so you can not match on
non-ASCII strings. But it is useful if you want to match
articles based on the raw header data. To enable it, set the
nnmail-mail-splitting-decodes variable to a non-nil value.
In addition, the value of the nnmail-mail-splitting-charset
variable is used for decoding non-MIME encoded string when
nnmail-mail-splitting-decodes is non-nil. The default
value is nil which means not to decode non-MIME encoded
string. A suitable value for you will be undecided or be the
charset used normally in mails you are interested in.

By default, splitting is performed on all incoming messages. If you
specify a directory entry for the variable mail-sources
(see Mail Source Specifiers), however, then splitting does
not happen by default. You can set the variable
nnmail-resplit-incoming to a non-nil value to make
splitting happen even in this case. (This variable has no effect on
other kinds of entries.)

Gnus gives you all the opportunity you could possibly want for shooting
yourself in the foot. Let's say you create a group that will contain
all the mail you get from your boss. And then you accidentally
unsubscribe from the group. Gnus will still put all the mail from your
boss in the unsubscribed group, and so, when your boss mails you “Have
that report ready by Monday or you're fired!”, you'll never see it and,
come Tuesday, you'll still believe that you're gainfully employed while
you really should be out collecting empty bottles to save up for next
month's rent money.